Preactivation conditioner for electroless metal plating system

ABSTRACT

A method is disclosed for treating the surface of a polymeric plastic substrate in a chemical plating process incorporating a preliminary chromic acid etch followed by one-step activation in a tin-palladium hydrosol. The method involves the step of immersing the substrate in a dilute aqueous acid solution of stannous chloride between the etching and activating steps.

PREACTIVATION CONDITIONER FOR ELECTROLESS METAL PLATING SYSTEM 3 Claims, No Drawings US. Cl 117/47 A, 117/71 R, 117/138.8 R, 117/160R, 106/1 lnLCl ..B44d 1/092, B44d 1/02 Field of Search 117/47 R,

[56] References Cited UNITED STATES PATENTS 3,011,920 12/1961 Shipler 1l7/2l3 3,212,918 10/1965 Tsu et a1. 117/47 X 3,245,826 4/1966 Luce et a1. 1 17/47 3,466,232 9/1969 Fracis et a]. 117/47 X Primary ExaminerAlfred L. Leavitt Assistant Examiner-Edward G. Whitby Attorneys-Steward and Steward, Merrill F. Steward and Donald T. Steward ABSTRACT: A method is disclosed for treating the surface of a polymeric plastic substrate in a chemical plating process incorporating a preliminary chromic acid etch followed by onestep activation in a tin-palladium hydrosol. The method involves the step of immersing the substrate in a dilute aqueous acid solution of stannous chloride between the etching and activating steps.

PREACTIVATION CONDITIONER FOR ELECTROLESS METAL PLATING SYSTEM This invention is concerned with metal plating of polymeric resin substrates and is more particularly directed to improvements in that portion of the metallizing process involving the laying down of an initial deposit or film of a desired metal, which deposit is continuous in its coverage of the substrate and is firmly bonded thereto. The art of metallizing plastics both for decorative and functional applications is undergoing rapid development. The general method employed consists of first chemically depositing on a polymeric plastic substrate a preliminary conductive metallic coating after which the substrate can then be electroplated by standard electrochemical techniques. The successful application of the initial conductive metallic layer is of crucial importance to the subsequent successful electroplating of one or more layers of the same or different metal in building up a total metallic deposit of desired thickness and finish characteristics. Copper, nickel, chromium and sometimes cobalt are the metals most commonly applied to commercial articles, and the process is used extensively in the automotive, appliance, plumbing and related industries today.

There are various methods in regular use for metallizing plastics. Most comprise a first or chemical, i.e. electroless, plating cycle of steps which consist of immersing the object to be plated in a number of aqueous or nonaqueous solutions, with each of such solutions contributing to a specific alteration of the polymer surface of the substrate to cause it to accept a thin and adherent initial metal layer. Thereafter a second processing phase or sequence of steps is involved during which thicker metal deposits are applied electrochemically, that is, by the application of an external source of electrical energy. In the course of these operations, the substrates or articles to be plated are sequentially subjected to the various treatment solutions, as by spraying them with or immersing them in such solutionsv in a commercial installation where mass production techniques are employed, the articles are generally supported from racks carried by endless conveyor means, by which the articles are progressed through the sequence of treatment steps, including both the chemical and electroless plating cycle as well as the final electroplating operations.

This invention is concerned primarily with improvements in the chemical or electroless plating cycle of operations, and it will be appropriate therefore, as background for the invention here disclosed, to consider first of all the sequence of operat ing steps involved in a typical, current procedure. in general, the steps in chemical plating of a polymeric plastic substrate comprise the following:

1. Clean the plastic part of surface grime, etc., in an aqueous alkali soak solution.

2. Contact the cleaned part with an organic solvent medium which may be either a single phase system or an admixed water-organic solvent emulsion. An example of such material and treatment is disclosed in a copending application, Ser. No. 645,90l,filed June 14, 1967, now U.S. Pat. No. 3,579,305.

3. Thorough water rinse of the part.

4. Contact of the part with an aqueous acid solution containing hexavalent chromium ions to etch the surface of the plastic. An example of such treatment is disclosed in U.S. Pat. No. 3,3 70,974; a different example is disclosed in a copending application, Ser. No. 474,198, filed June 25, 1965, now abandoned.

5. One or more rinses in water and/or solutions containing chromium-reducing or chromium-extracting agents. An example ofthis is disclosed in U.S. Pat. No. 3,563,784.

6. Contact the surface of the substrate with an acid tin-palladium hydrosol using a one-step" activator. An example is found in U.S. Pat. No. 3,01 1,920; also a further modification is disclosed in U.S. Pat. No. 3,532,518.

7. Again carefully rinse the surface.

8. Accelerate the activated surface of the plastic, using a dilute solution of an acid or alkali. Examples of suitable solu tions are shown in U.S. Pat. Nos. 3,011,920 and 3,532,5 l8.

9. Rinse in water.

l0. Immerse or otherwise contact the substrate surface with a chemical plating solution containing a reducible salt of the metal to be deposited on the surface. Examples are found in the patents listed in step 6 above; also in U.S. Pat. Nos. 3,2l2,9l8 and 3,370,974 for nickel and cobalt; also 3,095,309 for copper.

ll, Rinse the metallized surface in water, which is now ready for conventional electroplating.

While the foregoing general sequence of steps is employed commercially, there are certain deficiencies in the process which have manifested themselves, Small changes in operating procedures or ambient conditions tend to give rise to difficulties, the cause or reason for which is often unknown or hard to recognize since the art of plating on plastics is still largely empirical. This is particularly true in commercial operation where conditions are far less accurately controllable than in laboratory tests.

One of the most important problems is plating skip" or lack of continuity in the coverage of the substrate by the deposited metal. This is commonly noted on areas of the plastic which are highly stressed due to the method of molding the particular part. Plating skips often occur in other nonstressed areas also. Poor adhesion is another important problem, and often this does not show up until a late stage in the process, usually not until after completion of the metallizing procedure. All of this adds to the expense because the part must then be scrapped. Quite frequently these deficiencies arise from completely unrecognized conditions which further complicate the process.

It is known, of course, that any surface contamination of the substrate at the time it is introduced into the chemical plating solution plays an important role in the success of the plating operation and gives rise to the type of difficulties mentioned above. It is also known that hexavalent chromium, which is present in the usual etching step preceding activation, is a potent deterrent or poison to good deposition of the desired metal coating on the substrate. It is common practice, accordingly, to utilize after the etching step and prior to the usual activation step one or more thorough water rinses or other treatment solutions containing chrome-reducing agents to kill residual hexavalent chromium ions that may be present on the substrate. Obviously such preactivation chrome-kill treatments must not also act to impede the activation of the substrate surface. There is always, too, a problem of simple mechanical trapping of trace amounts of hexavalent chromium in crevices or other inaccessible areas where the substrate article has a complex shape or configuration.

it has now been discovered and it is a central concept of the present invention that many of the difficulties previously encountered with regard to skipping, discoloration and poor adhesion of chemically deposited metal plates on polymeric substrates using the one-step-type of activator solution can be reduced or eliminated by the use of a special preactivation conditioning step following the etching step and preceding the activation step in the cycle of plating operations. Briefly, it has been found that effective and more readily controllable metal plating of polymeric substrates can be accomplished by immersing or otherwise subjecting the etched substrate to a dilute aqueous acid solution of stannous chloride preceding the activation of the substrate in a one-step tinpalladium hydrosol activator system.

It is of course well known in the so called twostep sensitizing-activating procedure commonly employed in preparing plastic substrates for metal plating to immerse the substrate first in a dilute aqueous acid solution of tin chloride and then in a solution of palladium chloride. The adherent stannous chloride from the first dip acts to reduce the palladium chloride in the second dip to palladium metal, forming catalyzing foci on the surface of the substrate to promote deposition of the plating metal such as nickel, copper, etc., with which the substrate is subsequently covered. In the plating cycle where the activation of the substrate surface is accomplished by a single-step procedure using a tin-palladium hydrosol in which the palladium is in particulate or colloidal form, the procedure has of course been to simply wash the plastic carefully following etching in the sulfuric-chromic acid bath, and then subject it to the activating solution. The onestep activating system is often preferred to the older two-step activating system because it reduces bonding problems where the substrate presents both resin as well as metal surface areas, also it provides an advantage in that it may be operated at or near room temperature in contrast to elevated temperatures needed in the two-step system. But even in the one-step system a problem arises in effecting good metal coverage of and adhesion on stressed areas of the substrate plastic material, such as areas corresponding to the point or points of gating in the mold in which the substrate is extruded or otherwise formed. Here the etch solution employed can discolor the stressed area of the plastic solution with hexavalent chromium. This chromium, if not removed, will sometimes cause poor adhesion and poor metal coverage.

To overcome this condition, it has now been found that effective and more readily controllable plating of plastic substrates in the one-step activating procedure can be accomplished by dipping the substrate in an aqueous acid stannous chloride solution between the etching and one-step activating steps in the plating cycle. This does not introduce any additional operating step in the overall sequence of operations, inasmuch as it merely replaces the usual washing or rinsing step commonly employed between etching and activation. No separate rinsing is required after subjecting the substrate to the stannous preactivating solution, as the substrate may be transferred directly from such solution to the activator bath.

The invention is illustrated by the following example.

Example I A molded polypropylene substrate, previously cleaned of surface grime and grease in a mild aqueous alkali proprietary cleaner solution, is plated with a continuous, firmly adherent nickel deposit in the following manner:

a. lmmerse the substrate in an aqueous emulsion of an organic preconditioning agent comprising approximately, per liter of solution, 40 mls. of steam-distilled turpentine emulsified in water with surfactants. This emulsion and its prepara tion is more fully disclosed in copending application Ser. No. 654,901, filed June 14, 1967. Immersion of the part in this solution is maintained for approximately 5 minutes at a temperature of l50l60 F.

b. Cold water rinse.

c. Etch 5 minutes in an aqueous chromic-sulfuric acid bath at a temperature of about l75l80 P, such as the etchant solution disclosed in copending application Ser. No. 474,198, filed June 25, 1965.

d. lmmerse the substrate in an aqueous solution containing approximately 5 grams per liter of stannous chloride and sufficient hydrochloric acid to provide a pH of not over 1.0. This solution is maintained at room temperature and the substrate is immersed in it for a period of l to 3 minutes.

e. lmmerse substrate in an acid tin-palladium hydrosol ac tivator solution of the type disclosed in the aforesaid US. Pat. No. 3,532,518, for a period of about 3 minutes at room temperature.

f. Cold water rinse.

g. lmmerse the activated substrate for l to 3 minutes at room temperature in an aqueous acid accelerating solution,

such as a 10 percent perchloric acid solution.

h. Cold water rinse.

i. lmmerse substrate in a commercial nickel plating bath, for example for 5 minutes at a bath temperature of 90 F in an electroless nickel bath of the composition described in French Pat. No. l,497,35 9 of Frances E. Costello.

j. Cold water rinse.

K. Electroplate using conventional procedures.

The resulting nickel deposit is smooth, bright and completely continuous in coverage of the substrate, including such difficulty platable areas as locations corresponding to the gating points in the mold, or where the surface configuration of the substrate produces deep crevices or relatively inaccessible pockets. There is no discoloration of the nickel deposit in such stressed areas.

Copper plating of the substrate in place of nickel can similarly be effected with equally good results simply by substituting a commercial electroless copper plating solution for the nickel in the foregoing cycle of operations, all other steps being unchanged. The system is also effective for electroless plating of cobalt using any of the commercially available electroless cobalt plating solutions.

It has been shown that the preactivation conditioning step is operative for stannous chloride levels of as low as lOO parts per million in solution, and there appears to be no upper limit of the concentration of the stannous chloride so far as operability is concerned. However economic considerations govern the upper limit of this concentration since there is a higher replenishment cost occasioned by drag out. Accordingly, practical operating limits for the concentration of the stannous solution are on the order of at least ppm. to approximately 5 grams per liter. Stannous chloride is generally employed because of its greater availability but obviously other soluble stannous salts will serve equally as well. The acidity of the preactivating solution is maintained at a level of pH 0.5 to 1.0 by hydrochloric acid.

The preactivating conditioning procedure here disclosed is applicable in the one-step activation system of electroless plating of all of the usual platable plastic substrates currently in use, including not only polypropylene mentioned above but such other resins as phenolic, epoxy and polysulfone polymers, as well as copolymers such as acrylonitrile-butadiene-styrene.

What is claimed is:

1. In a process of chemically plating the surface of a nonconductive substrate with a metallic film selected from the group consisting of copper, nickel and cobalt, wherein the substrate surface is first etched in an acid hexavalent chromium solution followed by one-step activation in an acid tin-palladium hydrosol resulting from reduction of an aqueous hydrochloric acid solution of a palladium salt by a stannous salt and in which an excess of stannous ions is present, the step which comprises subjecting the etched substrate surface, immediately before said one-step activation, to contact with a dilute aqueous acid solution of a stannous salt.

2. The process as defined in claim 1, wherein the stannous salt is stannous chloride and the solution is acidified with hydrochloric acid to a pH of not greater than 1.0.

3. The process as defined in claim 2, wherein the stannous chloride is present in amount of from about 100 parts per million up to approximately 5 grams per liter. 

2. The process as defined in claim 1, wherein the stannous salt is stannous chloride and the solution is acidified with hydrochloric acid to a pH of not greater than 1.0.
 3. The process as defined in claim 2, wherein the stannous chloride is present in amount of from about 100 parts per million up to approximately 5 grams per liter. 